The usual alarm clock has a control member provided with a manually rotatable presetting element, such as a knob, and juxtaposed with a coacting member of the clockwork making not more than two revolutions every 24 hours. The two members are provided with mating formations that are aligned with each other in one predetermined relative angular position thereof, thus once every 12 or possibly every 24 hours, in which one of these members is shiftable by suitable biasing means (e.g. a spring) from a normal into an off-normal position to operate activating means for setting off the signal generators. The activating means may comprise a mechanical trigger or a switch closing an electrical circuit; in either case, the mating formations on the two juxtaposed members are generally so designed as to interengage with a snap fit when the selected time position is reached by the clockwork and thereafter gradually to disengage themselves by camming action as the coacting member moves out of its predetermined relative angular position.
Because of the backstop effect created by the snap fit, the control member cannot be reverse-rotated once the mating formations of the two members are interengaged. If, then, a user intending to delay the sounding of the alarm attempts such reverse rotation, damage to the mechanism could result. It has therefore been the practice to provide the presetting element of an alarm clock with a one-way brake or a ratchet coupling preventing backward rotation in any position of the clockwork. The drawback of such an arrangement is that a user desiring to postpone the preset alarm-release time by, say, a fraction of an hour must turn the control knob through a number of revolutions corresponding to an advancement of that time by more than 11 hours (or more than 23 hours in the case of a 24-hour clock).
This drawback also exists where, e.g. as described in German utility model No. 1,692,300, the control member is linked with its presetting element by a unidirectional coupling including a gear which transmits only forward rotation but swings out of mesh upon being rotated in reverse.